Sterile milk can be defined as milk free of micro-organisms which can grow under prevailing storing conditions. Microfiltration is one of a number of methods used for producing sterile milk. WO 98/57549 and U.S. Pat. No. 5,256,437 describe methods for the production of sterile milk using microfiltration. According to these methods, the milk is passed through a filter through which only particles smaller than a certain size can pass. This means that e.g. harmful microorganisms are filtered out. Filters used in microfiltration processes are often made of ceramic material.
During the production of sterile milk in this manner, the filter needs to be cleaned after a certain amount of time. As a rule, the system does not enter a cleaning phase in the midst of a production cycle; this only happens if the pressure that is required to maintain a constant flow through the filter exceeds a certain threshold value. This pressure increases gradually during a production cycle, as the filter pores become more and more fouled (i.e. clogged with particles filtered out from the milk).
The cleaning procedure is commonly called CIP (Cleaning In Place) and is performed by flushing the system with water with added detergents. A problem that sometimes occurs is that this cleaning fails; the cause of this might for example be a failure in the opening of a valve somewhere in the system. When the filters have not been completely cleaned before a production cycle starts, the cycle is shortened, since the above-mentioned pressure rises too fast. This leads to product losses, since the system has to enter the cleaning phase sooner than calculated.
To avoid this it would be desirable to have some way of testing whether or not the cleaning has been successful, before entering into a production cycle.